Optical sensor

ABSTRACT

An optical sensor assembly includes a light source, a surface plasmon-sensitive structure for reflecting light, a light detector, and a signal indicator. The light detector receives light that is reflected from the surface plasmon-sensitive structure at an angle which is sensitive to surface plasmon absorption.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical sensing and, more particularly,to an optical sensor which utilises the phenomenon of surface plasmonresonance.

2. Discussion of the Background

Surface plasmons are the quanta of a surface electromagnetic wavepropagating along the interface between a conducting material such as ametal and a dielectric. They represent the coupling of a longitudinaloscillation of the surface charge density with its associatedelectromagnetic fields. Surface plasmons can be excited by an electricfield of light if the component of the wave vector of the light wavealong the surface equals that of the same frequency surface wave. Theeffect is then called surface plasmon resonance (SPR) and may be seen asan absorption of the light. Kretschmann and Raether (Z. Naturforsch.,(1968) 23a, 2135) have produced a comprehensive description of thephenomenon. The effect is strongly dependent on the dielectricproperties at the metal surface and can therefore be used to sensematerials at or deposited on the metal surface or to sense changesbrought about in a previously existing overlayer by exposure to someother substance.

Some known forms of optical sensors utilise this phenomenon by scanningthe angle of incidence or by using the so called convergent beam method.In the latter approach polarised monochromatic or narrow band light isfirst expanded and then focused on to a metal film coated onto the backsurface of a glass prism or the front surface of a diffraction grating.The position of resonance is detected by using a linear diode array andthe source of radiation or light is usually a light emitting diode whichrequires polarisation filters for use as an SPR source or a polarisedgas laser such as a helium-neon laser.

SUMMARY OF THE INVENTION

Applicants are particularly, though not exclusively, interested inoptical sensors for sensing the presence of monoethylene glycol vapour(MEG). The UK gas distribution system for distributing natural gas topremises includes many miles of pipework with lead/yarn joints. In orderfor such joints to remain gas tight, the packing material (jute or hempfibres) must be maintained in a swollen state. Natural gas is a dry gaswhich tends to cause the packing material to dry out and shrink andpossibly result in some gas leakage at the joints. To prevent thisproblem occurring the natural gas is conditioned with MEG which isintroduced continuously in vapour form into the pipework by "fogging"units. The MEG serves as a yarn swellant. The maintenance of the correctlevel of MEG in the gas is important since too much results in dropletscondensing on the pipe walls and in MEG being wasted, while too littleresults in inadequate swelling of the packing yarn thereby increasingthe likelihood of some gas leakage at the joints. Thus monitoring of theconcentration of the MEG in the gas at various test points throughoutthe system is important.

An object of the invention is to provide an optical sensor whichutilises the phenomenon of surface plasmon resonance for sensing thepresence of an analyte in a fluid.

According to the invention an optical sensor for sensing the presence ofanalyte in a fluid comprises a light source; a surface plasmonresonance-sensitive device for reflecting light originating from thelight source and which device, on exposure to the analyte, responds soas to alter the intensity of the light which is reflected; a lightdetecting means for receiving light reflected from the device and forproducing in response to the received reflected light an output signalindicative of the presence of, or representative of the concentration ora concentration range of, the analyte in the fluid; control means forcontrolling the portion of the reflected light which is permitted toreach the light detecting means; and indicating means for receiving asaid output signal via the light detecting means and, in responsethereto, indicating the presence, concentration or concentration rangeof the analyte in the fluid. The fluid may be gas,vapour or liquid.

The control means may comprise one or more members defining or having anopening. The opening may be an aperture of fixed size defined by onemember. The member may comprise a thin planar member defining a singlevery small hole or a `pinhole`. Alternatively, the opening may be in theform of a slit defined by the one or more members. The control meansmay, alternatively, comprise an optical fibre with the bore therethroughconstituting the opening. Preferably, the opening is sufficiently smallto permit the passage therethrough of only a very small portion of thepart of the reflected light which has been more strongly influenced bythe SPR.

The light detecting means may be a photo-electric detecting means.Preferably, the photo-detecting means consists of one singlephotodetector, as opposed to a plurality or an array of discretephotodetectors.

The provision of a single small opening of fixed size and one singlephotodetector facilitates the construction of a small portable, compactunit which can be held in one hand during use.

Preferably, the light source is a diode laser. Diode lasers operatingboth in the visible and infrared parts of the spectrum can be used.

Since diode lasers produce polarised light a separate polarising filterdoes not have to be employed: thus simplifying the source optics and,again, facilitating the construction of a small unit.

The indicating means may be such as to indicate the presence of theanalyte only when the signal from the light detecting means isrepresentative of a concentration of analyte above a predeterminedminimum or threshold value. Thus, the indicating means may be arrangedto function in a simple on/off manner.

In another form of the sensor the indicating means may be calibrated soas to provide an indication of the concentration of the analyte in thefluid. For example, the indicating means may comprise two or morediscrete indicators, each one (when activated) being representative of adifferent concentration or range of concentrations of the analyte in thefluid.

The two or more discrete indicators may be activated in succession asthe signal from the light detecting means changes to be representativeof an increased concentration of the analyte in the fluid.

The indicating means and discrete indicators may comprise one or morevisual indicators such as light emitting diodes.

Alternatively, or in addition, the indicating means may produce anaudible sound when activated.

The sensor may comprise a second light detecting means for receivingfrom the device reflected light which is unaffected by the presence ofthe analyte and for producing in response to the reflected lightreceived a reference output signal, and means for comparing the outputsignal with the reference output signal and producing a resultant signalwhich serves as the input signal to the indicator means.

Preferably, the sensor is contained within a casing and is portable.Conveniently, the casing, and thus the sensor as a whole, issufficiently small to be supported on, and usable in, one hand.

In a preferred embodiment the sensing or sensitive surface of thesurface plasmon resonance-sensitive device is enclosed by an enclosurehaving an inlet means, via which fluid can enter the enclosure andcontact the sensing surface, and an outlet means via which the fluid canleave the enclosure. The sensor including the enclosure and its inletand outlet may be contained within the casing mentioned above.Preferably, means is provided for forcibly causing the fluid to enterthe inlet, pass through the enclosure and exit from the outlet.Conveniently, such means may be a fan or blower or pump located in theinlet or outlet.

In one embodiment, the light source, surface plasmon resonance-sensitivedevice, control means and light detecting means remain in permanentlyfixed predetermined positions, i.e. they are stationary or static withrespect to each other, while the sensor is being used. Thus the controlmeans and light detecting means are at set constant respective anglesrelative to the surface plasmon resonance-sensitive device. Scanning ofneither the incident light nor the reflected light is conducted. Theabsence of the need to scan and thus the absence of scanning means alsofacilitates the construction of a small unit.

However, alternatively, the light source and/or surface plasmonresonance-sensitive device and/or control means and/or light detectingmeans may be movable to or positionable in preset or predeterminedpositions preparatory to the sensor being used.

The surface plasmon resonance-sensitive device may comprise apropagating medium having a reflecting surface which supports or iscoupled to a metallic coating or layer which in turn is covered by orcoated with a film of material which is sensitive to the analyte, thatis the material is capable of adsorbing or absorbing the analyte. Forexample, the propagating medium may comprise a glass prism having aninternal face constituting the reflecting surface.

The reflecting surface may support the metallic coating directly.Alternatively, the reflecting surface may be coupled to a transparentbase member. In the latter arrangement the transparent base member maybe a glass slide, such as a glass microscope slide, which is coupled toa glass prism by means of a fluid having a refractive indexsubstantially matching the refractive indices of the glass of themicroscope slide and the glass of the prism. Where, for example, a glassprism and glass slide are used the fluid may be glycerol.

In an alternative embodiment of sensor, the reflecting surface may havethe form of a grating of suitable pitch with the metallic film beingsupported or formed directly on the grating.

The sensing material may be an organic material, for example,polypyrrole. Polypyrrole has been found by the present Applicants to beparticularly suitable when the sensor is for detecting the presence ofMEG in natural gas.

BRIEF DESCRIPTION OF THE DRAWING

In order that the invention may be more readily understood, referencewill now be made by way of example only, to the accompanying drawings,in which:

FIG. 1 illustrates schematically and in an idealised manner the kind ofeffect surface plasmon resonance produces in an optical sensor on whichthe sensor according to the invention is generally based,

FIG. 2 shows in somewhat schematic form one embodiment of sensoraccording to the invention,

FIG. 3 shows another embodiment of sensor according to the invention,

FIGS. 4a and 4b are graphs each illustrating the intensity and positionof light reflected from the SPR-sensitive device in the presence andabsence of analyte,

FIG. 5 shows a circuit diagram incorporating the photodetecting means inthe sensor in FIG. 2 or FIG. 3,

FIG. 6 shows a different circuit diagram incorporating another form ofphotodetecting means, and

FIG. 7 is a graph illustrating by way of example the effect of thepresence of a polypyrrole film on the light reflected from the surfaceplasmon resonance-sensitive device in the absence and presence of MEG.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, a diode laser 1 produces a beam of collimatedlight which passes through an optical lenses system 2 to produce aconverging beam 3 which passes into a prism 4 and is incident at theinternal face of reflecting surface 5. The outside of the prism at thereflecting surface is coated with a thin metal film 6, such as a goldfilm. Light is reflected in the form of a diverging beam 7. For lightincident at the internal face of the prism at a specific angle ofincidence (contained within the incident angle range of the convergingbeam) surface plasmon resonance is observed as a dip or reduction in theintensity of reflected light (when compared with reflected lightunaffected by the S.P.R. effect) as a result of absorption of lightincident at that specific angle due to the presence of the metal film.The region of the reflected beam over which SPR is observed is indicatedin FIG. 1 at 8.

In FIG. 2 the optical sensor 10 comprises a visible or infrared diodelaser module 11 and cylindrical lenses 12 and 13, all of which arerigidly mounted in a holder 14. The diode laser produces a collimated 15light beam which is formed in to a converging beam 16 and focused toprovide incident light on a SPR-sensitive device 17. The wave length ofthe light may, typically, be about 632 nm. The device 17 comprises aglass prism 18 having a side 19 which provides an internal reflectingsurface. In the applicants experiments the prism was made of BK7 glass.The external face of the side 19 is coated with a thin film, coat orlayer 20 of a suitable metal, such as gold having a thickness of,typically 40 nm. The gold, in turn, is coated or covered with a thinfilm 21 of polypyrrole having a thickness of, typically, 100 nm.Together, the gold and polypyrrole provide a region 22 which issensitive to the presence of MEG. The side of the prism bearing themetal layer 20 and polypyrrole film 21 forms one side of an enclosureconstituting a `gas cell` 23 to which is connected an inlet pipe 24 withan opening 25 via which gas or vapour can enter the gas cell and(influence or) contact the polypyrrole film 21. The gas cell 23 also hasconnected to it an outlet pipe 26 with an opening 27 via which the gasor vapour can leave the cell.

A pump 30 is located in an enlarged end portion 26a of the outlet pipe26 in order forcibly or positively to cause gas or vapour being sampledto pass into, through and out of the gas cell 23.

The reflecting region 22 of the SPR-sensitive device 17 causes theconverging incident light beam 16 to be reflected as a diverging beam 31in a manner similar to that indicated schematically in FIG. 1.

A `pinhole` aperture 33 in an otherwise opaque plate 32 determines theportion of the reflected diverging beam 31 which passes through theplate to be sampled and impinge (or be incident) on one singlephotodiode 34. The `pinhole` aperture may, typically, be about 100 to200 microns in diameter. The portion of the reflected diverging beamwhich is chosen to be sampled will be indicated below in thedescriptions of FIGS. 4A and 4B.

In FIG. 3, parts of this sensor which are equivalent to parts which havealready been described above with reference to FIG. 2 have beenidentified by the same reference numbers and will not be described anyfurther. In this embodiment an optical fibre 37 replaces the plate 32.The bore (not shown) of the fibre opens at 38 at one end thereof and thefibre is located by a holding means 39 such that the opening 38 is inthe same position as the pinhole 33 in the FIG. 2 embodiment. Thus theopening 38 controls or determines the portion of the reflected divergingbeam 31 which will be sampled and impinge on the one single photodiode34 to which the other end of the optical fibre 37 is connected in anysuitable manner such that only light emerging from the bore at the saidother end of the fibre impinges on the photodiode. The diameter of thebore of the optical fibre 37 is comparable to the diameter of the`pinhole` aperture in the previous embodiment, that is typically about100 to 200 microns.

The single photodiode 34 is connected to an indicating means 35 forindicating the presence of the analyte via electrical circuitry 36 whichwill be described below with reference to FIGS. 5 and 6. The circuitryis mounted on a printed circuit board, and the component parts of thesensor, including the circuit board, are housed in a small casing whichcan be held in one hand by a user. The casing also houses a compartmentfor a battery (not shown) for supplying electrical power to operate thesensor. In an alternative embodiment (not shown), the batterycompartment in the casing is omitted and a separate battery unit whichis electrically connectible to the circuitry in the casing is providedinstead.

In the embodiment of sensor shown in FIG. 2, the laser diode 11, prism18, apertured plate 32 and photodiode 34 are in permanently butadjustable fixed positions relative to each. Thus, the same portion ofthe reflected diverging beam will consistently be monitored by thephotodiode. The fixed relative positions of these components will havebeen determined as a result of previously conducted experiments in whichdifferent portions of the reflected beam, would have been observed withthe components in different relative positions in order to determine theoptimum positions. In the FIG. 3 embodiment, the end of the opticalfibre having the opening 38 is held in a single fixed predeterminedposition.

The basis on which the portion of the reflected beam to be monitored ischosen will now be illustrated with reference to FIGS. 4A and 4B.

When analyte is present and influences the sensitive film covering themetal film, the intensity of the reflected light at a fixed position inthe reflected beam at which SPR is observed differs from the intensitywhen the analyte is absent. The effect of SPR observed (in the absenceof analyte in the SPR sensitive layer over the metal film) may berepresented by the curve 40. On exposure of the SPR sensitive layer tothe analyte the effect may be represented by curve 41 in FIG. 4A and bya different curve 42 in FIG. 4B. In FIG. 4A the different positions ofthe curves 40 and 41 indicate that the range of angles of incidence overwhich resonance occurs has shifted but with relatively little change inthe size and shape of the curve and thus very little change in theintensity of the reflected light in respect of corresponding points onthe two curves. The `pinhole` aperture 33 and photo-detector 34identified in FIG. 2 are shown in FIGS. 4A and 4B (for illustrativepurposes) to indicate the fixed field of view (shown at 43) to which thephotodetector is exposed. This field of view is determined, at least inpart, by the size, shape and position of the `pinhole` aperture 33. Thephotodetector detects changes in reflected light intensity in the fieldof view within the range 44 resulting from the shifts caused bydifferent concentrations of the analyte in the gas or vapour.

In FIG. 4B the different positions of the curves 40 and 42 indicate thatthe range of angles of incidence over which resonance occurs has becomebroader with a lower peak absorption but with relatively little shifthaving regard to the positions of the two peaks of the curvescorresponding to the minimum intensities of reflected light. In thesecircumstances the photodetector 34 detects change in light intensity inthe field of view (shown at 45) within the range 46 resultingessentially from the changes in position of the minimum reflected lightintensities within the field of view caused by different concentrationsof the analyte in the gas or vapour.

The changes in the curves 41 and 42 shown in the graphs in FIGS. 4A and4B are illustrative of two extreme idealised situations. In practice anequivalent graph would show the relative position and shape of the curveto be intermediate to the corresponding curves 41 and 42 in FIGS. 4A and4B.

With reference to the electrical circuit 50 (which forms part of thecircuitry 36) in FIG. 5 the output signal from the photodiode 34 is fedto a Schmitt trigger 51.

The switching threshold is set by a potentiometer 52. The output fromthe trigger 51 is buffered by a voltage follower 53 which produces anoutput which is fed to the indicating means 35, comprising, for example,a piezoelectric sounder and/or a light emitting diode.

Based on experiments correlating the output from the photodiode with theconcentration of MEG in natural gas, the circuit is set such that theindicating means is activated when the concentration of MEG sensedexceeds a preset value.

With reference to the circuit 60 in FIG. 6, a reference photodiode 61 isprovided in addition to the photodiode 34. The reference photodiode ispositioned to receive a portion of the reflected beam which issubstantially unaffected by the SPR, for example at the positions shownin outline in FIGS. 2 and 3, to provide a constant reference outputsignal. In FIG. 3, the reference photodiode 61 receives light via anoptical fibre 66 having an end which is held in position by a holdingmeans 68 to locate the opening 67 of the bore (not shown) through theoptical fibre in the `reference` position. The output signals fromphotodiodes 34 and 61 pass to respective amplifiers 62 and 63 from whichrespective outputs pass to a means for comparing the signals comprisinga divider 64 which ratios the two sets of signals. The output from thedivider 64 passes to a calibration and driver circuit 65 which producesan output which forms the input to the indicating means 35. Theindicating means 35 may be a digital display device or an analoguedisplay device such as a light emitting diode bar graph array which iscalibrated in terms of the concentration of MEG in natural gas,utilising correlation between the output from the photodiode with theconcentration of MEG, as before.

The use of a diode laser, together with the converging incident beamtechnique and one single photodiode, as described in connection with theembodiment of sensor in FIG. 2, confer the considerable advantage ofenabling the production of a compact, lightweight, low power consumingsystem. Moreover the construction of the sensor is such that anglescanning is avoided.

The gold film was deposited directly on the external surface of theprism (or on the planar glass slide) by known vacuum evaporationtechnique.

The polypyrrole was laid down on the metal film by the followingelectrochemical technique. A conventional electrochemical potentiostatwas set up with the gold coated surface of the prism acting as thecathode onto which the film of polypyrrole was to be deposited. A goldcoated glass slide was used as the counter electrode and a calomelelectrode was used as the reference electrode. A solution containing 0.1molar concentration of pyrrole and 0.1 molar concentration of potassiumchloride with a phosphate buffer was used as the electrolyte. The cellcould be operated in either a constant potential mode or a cyclicvoltammetry mode. Both techniques were found to produce satisfactory anduseable films. The deposition potential was kept below 0.7 volts.Typical deposition times ranged from a few minutes to 10 minutes.

When exposed to monoethylene glycol such polypyrrole films give strongreversible SPR shifts which enable the films to be used as a sensingmedium for monoethylene glycol. FIG. 7 shows typical results for apolypyrrole film produced after five minutes deposition time.

Although in the above Example the fluid used was a gas or vapour, itwill be appreciated that the apparatus could be modified by a manskilled in the art so as to be usable with a liquid which contains theanalyte. Thus, the `gas cell` 23 may be modified, if necessary, so as tobe a liquid cell.

We claim:
 1. A process for using an optical sensor for sensing thepresence of monoethylene glycol vapor, said sensor comprising a lightsource for producing light, a surface plasmon resonance-sensitivestructure positioned to intercept the light and for reflectingintercepted light originating from the light source, wherein when saidsurface plasmon resonance sensitive structure is exposed to the analytemonoethylene glycol it responds so as to alter the intensity of theintercepted light which it reflects, a light detecting means forreceiving light reflected from the surface plasmon resonance sensitivestructure and for producing an output signal in response to the receivedreflected light indicative of the presence of, or representative of theconcentration or a concentration range of, the analyte in the fluid,control means for controlling the portion of the reflected light whichis permitted to reach the light detecting means, indicating means forreceiving the signal and in response thereto indicating the presence,concentration or concentration range of the analyte in the fluid, andwherein said surface plasmon resonance-sensitive structure comprises ametallic surface and a film of polypyrrole on the metallic surface, saidprocess comprising the steps of:exposing the sensor to a stream of gas;and determining the quantity of monoethylene glycol vapor in the streamof gas.
 2. A process according to claim 1, further comprising the stepsof:controlling the value of monoethylene glycol vapor in the stream ofgas to maintain a preferred value of monoethylene glycol vapor in thestream of gas based upon the determined value of monoethylene glycol inthe stream of gas.
 3. A process according to claim 2, wherein saidstream of gas is a stream of natural gas.
 4. A process for using anoptical sensor for sensing the presence of vapor, said sensor comprisinga light source for producing light, a surface plasmonresonance-sensitive structure positioned to intercept the light and forreflecting intercepted light originating from the light source, whereinwhen said surface plasmon resonance-sensitive structure is exposed tothe analyte monoethylene glycol it responds so as to alter the intensityof the intercepted light which it reflects, a light detecting means forreceiving light reflected from the surface plasmon resonance-sensitivestructure and for producing an output signal in response to the receivedreflected light indicative of the presence of, or representative of theconcentration or a concentration range of, the analyte in the fluid,control means for controlling the portion of the reflected light whichis permitted to reach the light detecting means, indicating means forreceiving the signal and in response thereto indicating the presence,concentration or concentration range of the analyte in the fluid, andwherein said surface plasmon resonance-sensitive structure comprises ametallic surface and a film on the metallic surface whose opticalabsorption properties change upon exposure to monoethylene glycol, saidprocess comprising the steps of:exposing the sensor to a gas; anddetermining the concentration of the monoethylene glycol vapor in thegas.
 5. A process according to claim 4, further comprising the stepsof:controlling the concentration of monoethylene glycol vapor in the gasto maintain a preferred concentration of monoethylene glycol vapor inthe gas based upon the determined concentration of monoethylene glycolin the gas.
 6. An optical sensor assembly for sensing the presence ofanalyte in a fluid, comprising:a light source for producing light; asurface plasmon resonance-sensitive structure for reflecting the lightproduced by the light source which, when exposed to the analyte,responds so as to alter the intensity of light that it reflects; a lightdetecting means for receiving light reflected from the surface plasmonresonance-sensitive structure and for producing an output signal inresponse to the received reflected light which output signal isindicative of the presence of, or representative of the concentration ora concentration range of, the analyte in the fluid; control means forcontrolling the portion of the light reflected by the surface plasmonresonance-sensitive structure which reaches the light detecting means;indicating means for receiving the signal and in response theretoindicating the presence, concentration or concentration range of theanalyte in the fluid; and wherein said surface plasmonresonance-sensitive structure comprises a metallic surface and a film ofpolypyrrole on the metallic surface.
 7. An optical sensor assembly forsensing the presence of analyte in a fluid, comprising:a light sourcefor producing light; a surface plasmon resonance-sensitive structure forreflecting the light produced by the light source and which structure,on exposure to the analyte, responds so as to alter an intensity of thelight produced by the light source which is reflected; a light detectingmeans for receiving the light reflected from the surface plasmonresonance-sensitive structure and for producing in response to thereceived reflected light an output signal indicative of the presence of,or representative of the concentration or a concentration range of, theanalyte in the fluid; control means for controlling the portion of thereflected light which is permitted to reach the light detecting means;and indicating means for receiving the output signal and in responsethereto indicating the presence, concentration or concentration range ofthe analyte in the fluid, wherein the surface plasmonresonance-sensitive structure comprises a propagating medium having areflecting surface which supports or is coupled to a metallic layer andthe metallic layer is covered by a film of material which is sensitiveto the analyte, and in which the film of material which is sensitive tothe analyte is polypyrrole.